Original link: Introducing Pyrefly: A new type checker and IDE experience for Python / Facebook.
It may be a useful tool.
I don't see it is particular interesting that it is written in Rust. I suspect that the primary reason for chosing Rust was not that Rust is the best language for such a tool but that the engineers wanted to try Rust for something. What makes Rust unique is memory safety withour garbage collection, but that does not seem important for such a tool.